1. Field of the Invention
The present invention relates to a photoelectric converting device capable of accumulating photo-excited carriers and controlling output by thus accumulated voltage.
2. Related Background Art
FIG. 1(A) is a plan view of a photoelectric converting device disclosed in the European Patent Application Laid-open No. 132076, FIG. 1(B) is a cross-sectional view along a line I--I therein, and FIG. 1(C) is an equivalent circuit diagram of a photosensor cell.
In these drawings there are shown photo-sensor cells arranged on an n-silicon substrate 101, and each photosensor cell is electrically insulated from neighboring cell by a separating area 102 composed of SiO.sub.2, Si.sub.3 N.sub.4 or polysilicon.
Each photosensor cell is constructed in the following manner.
On an n-type area 103 with a low impurity concentration formed for example of an epitaxial process, there is doped a p-type impurity to form a p-type area 104, in which there is formed an n.sup.+ -type area by impurity diffusion or ion implantation. The p-type area 104 and n.sup.+ -type area 105 respectively constitute a base and an emitter of a bipolar transistor.
On such n-type area 103, including the above-mentioned other areas, there also formed an oxide layer 106 on which provided a capacitor electrode 107 of a determined area. Said capacitor electrode 107 faces the p-type area 104 across the oxide layer 106 to constitute a capacitor C.sub.ox. The potential of the floating p-type area 104 is controlled by the application of a pulse voltage to said capacitor electrode 107
There are further formed an emitter electrode 108 connected to the n.sup.+ -type area 105, a wiring 109 for reading the signal from the emitter electrode 108, a wiring 110 connected to the capacitor electrode 107, an n.sup.+ -type area 111 of a high impurity concentration formed on the rear side of the substrate 101, and an electrode 112 for providing a potential to the collector of the bipolar transistor.
In the following, there will be explained the basic function of the define. It is assumed that the p-type area 104, constituting the base of the bipolar transistor, is at a negative initial potential. When light 113 enters said p-type area 104, a charge corresponding to the amount of light is accumulated therein (accumulating step). The base potential varies by the thus accumulated charge, and the emitter-collector current is controlled by said potential variation. Thus, an electrical signal, corresponding to the amount of incident light, is read from the floating emitter electrode 108 (read-out step). For eliminating the charge accumulated in the p-type area 104, the emitter electrode 108 is grounded and a refreshing positive pulse is applied t the capacitor electrode 107. Said application of positive voltage creates a forward bias for the p-type area 104 with respect to the n.sup.+ -type area 105, whereby the accumulated charge is eliminated. At the end of said refreshing pulse, the p-type area 104 returns to the initial negative potential state (refreshing step). Thereafter the above-described accumulation, readout and refreshing steps are repeated.
In summary, the above-described device is to accumulate the charge, generated by incident light, in the p-type area 104 constituting the base and to control the current flowing between the emitter 108 and the collector 112 by the thus accumulated charge. Consequently the accumulated charge is read after amplification by each cell, and it is rendered possible to achieve a high output, a high sensitivity and a low noise level.
The potential Vp generated in the base by the holes accumulated therein by photoexcitation is given by Q/C, wherein Q is the amount of charge accumulated in the base while C is the capacity connected to the base. As will be apparent from this equation, the potential Vp generated by photoexcitation remains almost constant in case of a high degree of integration since Q and C become smaller with the reduction in cell size. Consequently the above-described method is advantageous also for achieving a high resolution in the future.
On the other hand, the change in the base potential Vb during the application of the refreshing positive voltage in the refreshing step can be determined from the following equation: ##EQU1## Wherein Cbe is the base-emitter capacity, Cbc is the base-collector capacity, and Ib is the base current.
FIG. 2 shows the time-dependent change of the base potential Vb during the application of the refreshing positive voltage.
In this chart, the initial base potential at the start of application of the refreshing pulse depends on the magnitude of the accumulated voltage Vp, since the application of a refreshing positive pulse to the capacitor electrode 107, in a state in which the initially negative base potential has been elevated by an accumulated voltage Vp in the accumulating step, elevates the initial base potential by said accumulated voltage Vp.
Also as shown in said chart, the time of retention of the initial base potential varies according to the magnitude of said initial base potential, but, after the lapse of said time, the base potential Vb lowers in the same manner regardless of the initial base potential. Consequently, if the refreshing time t is sufficiently long, the base potential Vb can be reduced almost to zero regardless of the magnitude of the accumulated voltage Vp, so that the base potential Vb can be returned to the initial negative state at the end of the refreshing pulse.
In practice, however, in order to achieve a high-speed operation, the refreshing operation is completed when the base potential Vb reaches a value Vk at a refreshing time t=t.sub.0. Even if the base potential Vb has a certain retentive value, it is possible to reduce the base potential Vb to a constant negative potential at the end of the refreshing pulse if the base potential Vb is always equal to the value Vk at the refreshing time t=t.sub.0, and to take said negative potential as the initial state.
However, in the conventional photoelectric converting device, there will result a progressive lowering of the retentitive potential Vk by repeated refreshing operations, thus resulting in an after-image effect.
As an example, it is assumed, in FIG. 2, that the initial base potentials are 0.8V and 0.4V respectively in a high-intensity illuminated cell and a low-intensity illuminated cell. After the lapse of the refreshing time t.sub.0, the base potential Vb of the former cell reaches the determined retentive potential Vk but that of the latter cell reaches a potential Vl lower than Vk. If the refreshing pulse is terminated at this point, the base potential Vb of the low-intensity illuminated cell becomes lower than the initial negative potential, and the accumulating and read-out steps are initiated from said lower potential. Thus, if the refreshing operation is repeated under a low illumination intensity, the retentive base potential gradually lowers, so that there can be obtained an output lower than that corresponding to the amount of incident light even after receiving a high illumination intensity in this state. In this manner there results an afterimage effect.
Such phenomenon is presumably due to the deficiency in positive holes caused by recombination in the base area induced by repeated refreshing operations. Consequently the afterimage effect becomes apparent if a low illumination state continues where the deficient positive holes cannot be replenished.